In an exemplary printing press such as a web offset press, a web of material, typically paper, is fed from a storage mechanism, such as a reel stand, to one or more printing units that repetitively imprint the web with images. The imprinted web is typically driven through a number of processing units such as a dryer unit, a chill stand, and possibly a coating machine. The web is then typically fed to a former/folder to be slit, folded, and cut into multipage signatures.
It is desirable to monitor the quality of the imprinted web, to ensure that the amount of applied ink is appropriate and produces the desired optical characteristics, and to ensure that the different ink colors are properly aligned (registered) with respect to one another. Further, monitoring the web is important to ensure that the imprinted web does not include defects such as ink blots, lack of ink in areas where ink should be, smears, streaks, or the like, and to insure that various print processes occur at a correct location with respect to the ink on the web. For example, ink color control systems, color registration systems, and defect detection systems are known systems used in connection with monitoring the quality of the imprinted web. Various other types of control systems are also known for controlling the position of the web with respect to a processing unit of the printing press. For example, a cutoff control system operates to control the longitudinal position of the web so that the cutting of the web into signatures occurs at a desired location.
Such systems generally include an imaging assembly for obtaining image data from a portion of the moving imprinted web. Typically, the acquired image data is compared to reference image data. The resultant information is used, for example, to control the amount of ink applied to the web, the alignment of the printing plates with respect to each other, to mark or track the whereabouts of resultant defective printed product, or to control the location of the imprinted web with respect to a processing unit.
More specifically, in a typical ink color control system for controlling the amount of ink applied on a printing press, the camera collects image data representative of color patches printed on the web. These patches generally extend across the width of the web. Pixels of the color patch image data are then processed, and assigned a color value that is compared against a desired color value. If the absolute difference between the desired color value and the determined color value for a number of pixels in an ink key zone is outside a predetermined tolerance, an associated ink key is then controllably adjusted to effect a change in the ink flow rate. Markless color control systems are also known that do not require the use of separate color patches but instead measure color values in the desired graphical/textual printed work itself. Examples of ink color control systems are described in U.S. Pat. Nos. 5,967,049 and 6,318,260.
A typical defect detection system also acquires an image of the imprinted web. The acquired image is subsequently compared to a stored digital template image. Any discrepancy between the acquired image and the template image beyond some tolerance is considered to be a defect. The defects are then logged in a data file, and can be categorized as isolated defects or non-isolated defects. Non-isolated defects occur when the system detects a change in color due to a change in inking level over a large portion of the web. When non-isolated defects are reported, an alarm will subsequently be set off to alert an operator to take appropriate corrective action. Isolated defects can be tracked such that the associated printed products are marked as defective, or are otherwise separated from the acceptable printed products.
Typically, color registration systems also compare acquired image data to reference image data and adjust the registration or alignment of each ink color with respect to the others by adjusting the positions of the printing plates with respect to each other. Color registration systems using marks or patches are known, as are markless systems. Examples of such systems are described in U.S. Pat. Nos. 5,412,577 and 5,689,425.
These control systems all require image data to be acquired from the printed work on the web, and vary in the amount and resolution of data required. For example, to detect defects in the entire printed work, it is desirable to acquire image data for the entire width of the web, as well as the entire length of the web. An ink key control system, because it controls ink keys across the lateral extent of the web, would preferably obtain image data from patches (or the desired printed work itself) across the entire width of the web, but only once per image repeat. Similarly, a color registration system using color marks would obtain image data only once per image repeat. Additionally, marks for color registration or cutoff control generally do not extend across the web.
Typical imaging assemblies include lighting elements for illuminating the web, and a camera having sensors for sensing light and optical elements for focusing light reflected from the imprinted web to the sensors. Known sensors include area array sensors having two-dimensional arrays of sensing elements, and line scan sensors, which include a single line of sensing elements aligned across the web. With line scan sensors, two dimensional image data is obtained by acquiring successive lines of data as the imprinted web moves with respect to the line sensors.
Typical optical elements are lenses that reduce the image on the web in order to obtain a desired resolution for the image data. This typically results in a field of view for the camera that is several inches in width. With such prior art imaging assemblies, the distance between the web and the camera generally needs to be comparable to the width of the web being imaged. Thus, prior art imaging assemblies for printing presses generally require a distance on the order of approximately four feet between the web and the camera. Further, because the cameras themselves were often expensive, prior art systems typically minimized costs by using a single camera with a positioning unit to move the imaging assembly across the width of the web.